This invention relates to a mold for molding golf balls. More particularly, in connection with a golf ball comprising a core, an intermediate resin layer, and a cover, the invention relates to a mold defining therein a spherical cavity for molding the resin layer.
In the golf ball art, a number of attempts have been made for accomplishing improvements in both the distance and feel of golf balls. As to solid golf balls comprising a solid core and a cover, it is a common practice to interpose one or more resin layers between the core and the cover to form a multilayer structure and adjust the hardness and dimensions (e.g., diameter and gage) of the resin layer.
While the resin layer can be formed by several methods, typical methods are a method of injection molding a resin material around a core in a mold and a compression molding method involving preforming resin sheets into a pair of hemispherical cups, encasing a core in the cups, and molding the cups in a mold under heat and pressure. The injection molding method is simple and preferable. The injection molding method, however, has the misalignment problem that the core can be off-centered during molding. A slight amount of misalignment can have a serious influence on the golf ball because the golf ball with off-centered core is inferior in performance when hit.
For the injection molding of the resin layer, there are many limits with respect to flow and thickness. The molding technology cannot fully accommodate for the recent development of resin layers including the advent of a new material different from the conventional resin materials and a significant change of the thickness.
Under the circumstance that golf balls of multilayer structure are desired, the molding method still encounters difficulty in forming a thin resin layer of uniform thickness around a spherical core. That is, the molding method fails to come up with the development of resin layers.
In connection with a golf ball of multilayer structure having a resin layer between a core and a cover, an object of the invention is to provide a mold defining a spherical cavity having projections for molding the resin layer, the mold having the advantages of minimized misalignment of the core, minimized formation of projection marks, an ability to form a thin layer of uniform thickness, and an ability to comply with a multi-ply resin layer. Another object of the invention is to provide a golf ball comprising the resin layer formed using the mold and having a stabilized structure and improved durability against strikes.
The invention generally pertains to a golf ball of multilayer structure having a resin layer between a core and a cover. Specifically, the invention is directed to a mold for molding the resin layer around the core which is placed in the mold cavity. It is desired to mold the resin layer of uniform thickness while preventing the core from being misaligned during molding. The inventor paid attention to projections on the cavity wall for supporting the core. It has been found that the resin layer of uniform thickness can be formed while minimizing formation of projection marks and preventing misalignment of the core if the total of the cross-sectional areas of the projections divided by the surface area of the cavity wall and the total of the volumes of the projections divided by the volume of the space between the core and the cavity wall are controlled within specific ranges.
It has further been found that by specifying the maximum height of projections, determining the height of projections on the basis of the diameters of the cavity and the core, specifying the shape of projections, adjusting the cross-sectional area of projections in their axial direction, and/or optimizing the number of projections, molding conditions are improved so that the resin layer which exerts its modification effect to a full extent can be formed.
It has also been found that in a golf ball comprising a core, a resin layer and a cover, when the resin layer is molded using the above-described mold, the resulting resin layer becomes uniform. This ensures that the ball takes full advantage of the modification effect of the modified resin compound, has a stabilized structure, and becomes fully durable against strikes.
In connection with a golf ball comprising a core, a resin layer enclosing the core, and a cover enclosing the resin layer, a first aspect of the invention provides a mold having a cavity wall defining a spherical cavity, the core being placed in the cavity to define a space for molding the resin layer. The cavity wall is provided with a plurality of projections extending toward the center of the cavity. The total of the cross-sectional areas of the projections at their base divided by the surface area of the cavity wall is up to 2%. The total of the volumes of the projections divided by the volume of the space is up to 4%.
Preferably the projections have a height of (D1-D2)/2 (mm) wherein D1 is the diameter (mm) of the spherical cavity and D2 is the diameter (mm) of the core to be placed therein. The projections are typically cylinder or prism shaped or have a cross-sectional area that gradually decreases toward the center of the cavity. The number of projections is preferably up to 20.
In a second aspect, the invention provides a golf ball comprising a core, a resin layer enclosing the core, and a cover enclosing the resin layer, the resin layer being molded using the above-defined mold.